题解 | #求二叉树的层序遍历#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 * }
 */

public class Solution {
    
     ArrayList<ArrayList<Integer>> res = new ArrayList<>();
    public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
        if (root == null) {
            return res;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            ArrayList<Integer> list = new ArrayList<>();
            int n = queue.size();
            for (int i = 0; i < n; i++) {
                TreeNode cur = queue.poll();
                list.add(cur.val);
                if (cur.left != null) {
                    queue.add(cur.left);
                }
                if (cur.right != null) {
                    queue.add(cur.right);
                }
            }
            res.add(list);
        }
        return res;
    }
}